Die cast magnesium alloy

ABSTRACT

A magnesium based alloy contains zinc, aluminium, calcium and/or beryllium, optionally manganese, and the balance magnesium except for incidental impurities. The zinc and aluminium contents fall within a quadrangle defined by lines AB, BC, CD and DA and the calcium and beryllium contents fall within a quadrangle defined by lines EF, FG, GH and HE wherein: A is 10% Zn-2.5% Al, B is 10% Zn-5% Al, C is 13% Zn-6.4% Al, D is 19% Zn-2.5 % Al, E is 0.01% Ca-0% Be, F is 1% Ca-0% Be, G is 0% Ca-0.0025% Be, and H is 0% Ca-0.0001% Be.

FIELD OF THE INVENTION

The present invention relates to magnesium/zinc/aluminium (Mg—Zn—Al) alloys which contain small amounts of calcium and/or beryllium.

BACKGROUND TO THE INVENTION

Due to their excellent strength to weight ratios, magnesium alloys are well recognised as commercially desirable materials. The most commonly used magnesium alloy is AZ91 which contains about 90% magnesium, 9% aluminium and 1% zinc. On a weight basis, zinc is about 65% of the price of magnesium and hence magnesium alloys of increased zinc content would be desirable provided that they exhibited commercially satisfactory properties.

A serious disadvantage of using magnesium alloys is the danger of ignition of molten alloy. Magnesium alloys which are sufficiently resistant to oxidation to obviate the need for protective cover gases or the like when molten alloy is exposed to air would be advantageous.

U.S. Pat. No. 2,380,200 (Stroup et al) which issued in 1945 relates to magnesium base alloys and to methods of preventing oxidation of magnesium and magnesium base alloys. The patent notes that:

“Its general object is the provision of improvements which mitigate difficulties arising from the propensity of magnesium to oxidise when in contact with air, moisture or with other media containing oxygen. Commercial usefulness of a metal, such as magnesium, depends not alone upon its essential properties, or those which may be imparted to it by alloying it with lesser quantities of other metals, but also upon the ease with which the metal, or such alloys, may be remelted, cast, worked or otherwise formed into the various conditions and shapes necessary to ultimate use. The propensity of magnesium to destructively oxidise when in the molten state is great. Under many conditions, normal to the handling of other molten metals, molten magnesium burns or otherwise reverts to the oxide in very substantial part. When in the solid state, magnesium base alloys oxidise, under some conditions, to a comparatively severe extent. Since extensive handling of magnesium and magnesium base alloys in the molten condition is a necessary preliminary to operations designed to shape or work the metal, the difficulties presented by this pronounced tendency to oxidise are encountered in almost every instance and are universal in the magnesium industry.”

“Confronted with these problems the industry has devised methods and devices by which to shield molten magnesium and magnesium base alloys from contact with air and moisture, or other deleterious media, during manufacturing operations. One such method is to envelop the molten metal in a protective gas. Another is to constantly protect its exposed surfaces with a salt flux. Other more elaborate methods and devices are frequently necessary. Other means have also been sought to minimise the tendency of magnesium and magnesium base alloys to oxidise and to thus reduce the necessity for the expensive protective measures above mentioned. Calcium has been alloyed with the magnesium for this purpose, and while the magnesium or magnesium base alloy thus alloyed does not oxidise as severely as before the total effect is not sufficient to do more than supplement the usual protective measures. Better results have been obtained when beryllium has been added to magnesium or magnesium base alloys, it having been found that the effect of beryllium in minimising the oxidation of magnesium is much greater than that of a corresponding amount of calcium.”

U.S. Pat. No. 4,543,234 (Foerster) relates to Mg—Al—Zn—Si—Mn alloys containing 0.0025-0.0125% dissolved beryllium “to inhibit burning, with the amount of beryllium being increased with increasing oxygen content of the atmosphere.” U.S. Pat. No. 4,543,234 also notes that “a beryllium content of on the order of 0.001 percent is considered to be inadequate for the purpose of inhibiting excessive oxidation of the molten magnesium.”

A paper entitled “Characterization of the oxidation surface layer on non-combustible Ca-bearing Mg melts” by M. Sakamoto, S. Akiyama and K. Ogi, presented at the 4^(th) Asian Foundry Congress, 27^(th)-31^(st) Oct. 1996, reported the ignition temperatures of calcium containing magnesium base alloys (see FIG. 2 in the paper). The measured ignition temperatures varied considerably between repeats of the same alloy composition. In most of these repeats, alloys with 0.5% or more calcium did not ignite until the melting point of the alloy was exceeded; however, instances are shown of ignition occurring below the melting point for calcium levels as high as 4%.

U.S. Pat. No. 5,855,697 (Luo et al) relates to a magnesium alloy having superior elevated temperature properties and is not concerned with oxidation suppression. U.S. Pat. No. 5,855,697 notes that calcium addition is known to improve the high-temperature strength and creep resistance and that calcium contents of 0.2% by weight and greater are desirable. It is further noted that such calcium additions severely deteriorate castability rendering the alloy incapable of being cast by conventional die casting processes. U.S. Pat. No. 5,855,697 teaches that the castability of a magnesium-aluminium-calcium alloy can be restored by inclusion of zinc. A zinc content of about 6 to about 12 weight %, more preferably about 6 to about 10 weight %, is taught and the “upper limit of the zinc range is set at about 12 weight %, more preferably, about 10 weight % so that the density of the alloy remains low.” The presence of zinc is said to enable calcium to “be added in amounts up to 2 weight %, preferably up to 1.5 weight %, in order for the alloy to achieve the maximum creep resistance while maintaining good die-castability.”

U.S. Pat. No. 5,855,697 exemplified the below listed alloys. Accordingly, U.S. Pat. No. 5,855,697 does not exemplify an alloy containing more than 8.15% Zn.

-   Mg-5% Al-8% Zn with Ca contents ranging between 0 and 2% (see FIGS.     2 and 3 of U.S. Pat. No. 5,855,697) -   Mg-5% Al-1% Zn with Ca contents ranging between 0 and 2% (see FIGS.     2 and 3 of U.S. Pat. No. 5,855,697) -   Mg-4.57% Al-8.15% Zn-0.23% Ca-0.25% Mn (see Table 1 of U.S. Pat. No.     5,855,697) -   Mg-4.74% Al-8.12% Zn-0.59% Ca-0.25% Mn (see Table 1 of U.S. Pat. No.     5,855,697) -   Mg-4.67% Al-8.12% Zn-1.17% Ca-0.27% Mn (see Table 1 of U.S. Pat. No.     5,855,697)

SUMMARY OF THE INVENTION

The present invention provides an alloy consisting of:

zinc (Zn) and aluminium (Al) in amounts which fall within a quadrangle defined by lines AB, BC, CD, and DA wherein:

A is 10% Zn-2.5% Al,

B is 10% Zn-5% Al,

C is 13% Zn-6.4% Al, and

D is 19% Zn-2.5% Al;

calcium (Ca) and/or beryllium (Be) in amounts which fall within a quadrangle defined by lines EF, FG, GH and HE wherein:

E is 0.01% Ca-0% Be,

F is 1% Ca-0% Be,

G is 0% Ca-0.0025% Be, and

H is 0% Ca-0.0001% Be optionally Mn; and

the balance Mg except for incidental impurities. Unless otherwise stated, all percentages in this document are % by weight.

The quadrangle defined by lines AB, BC, CD, and DA is illustrated in FIG. 1 which is a plot of aluminium v zinc content. The quadrangle defined by lines EF, FG, GH and HE is illustrated in FIG. 2 which is a plot of beryllium v calcium content.

All alloys of the present invention contain a minimum of 10% zinc, preferably greater than 11% zinc, more preferably greater than 12% zinc, more preferably about 12-14% zinc, and most preferably about 12-13% zinc. Most surprisingly, the present inventor has ascertained that such zinc additions suppress the ignition of the alloy in the molten state in the absence of alkaline earth elements such as beryllium or calcium. Without wishing to be bound by theory, the ignition suppression is believed to be a consequence of the vapour pressures of magnesium and zinc and the amount of zinc present in the alloys.

The vapour pressures of zinc and magnesium above a molten alloy can be calculated using information from a paper entitled “Vapour Composition and Activities in Mg—Zn Liquid Alloy at 923K” by K. T. Jacob, S. Srikanth and Y. Waseda in Thermochimica Acta, 1988, vol 130, pages 193-203. The ratio of the vapour pressure of zinc relative to the vapour pressure of magnesium increases rapidly as the amount of zinc in the molten alloy is increased. A molten alloy containing 10% by weight of zinc and 90% by weight of magnesium is calculated to produce a vapour containing 22% by weight of zinc and 78% by weight of magnesium. Without wishing to be bound by theory, the zinc vapour is believed to interfere with ignition of the magnesium vapour.

Although molten alloys containing more than 10% zinc resist ignition, they tend to form a blackened layer on the surface of a solidified sample. The addition of a small amount of calcium and/or a small amount of beryllium has been found sufficient to result in a shiny surface appearance when solidified. As little as 0.01% calcium or as little as 0.0001% beryllium have been found sufficient in combination with zinc and aluminium contents in accordance with the present invention to produce this effect. Without wishing to be bound by theory, the shiny surface appearance is believed to be a consequence of an enrichment in the calcium and/or beryllium content of the oxide layer formed on the surface of the melt.

When present, the calcium content is preferably 0.01-0.5%, more preferably 0.01-0.3%, more preferably 0.02-0.3%, more preferably 0.05-0.3%, more preferably 0.05-0.2%, more preferably 0.05-0.15%, most preferably about 0.1%. Calcium contents in excess of 1% are undesirable because they have been found to diminish the mechanical properties of the alloys and cause die soldering when die cast.

When present, the beryllium content is preferably 0.0002-0.0025%, more preferably 0.0002-0.002%, more preferably 0.0005-0.002%, more preferably 0.0005-0.0015%, more preferably 0.0005-0.001%, most preferably about 0.0008%. Beryllium contents in excess of 0.0025% are unnecessary in order to obtain the desired effect. In view of beryllium's toxicity it is therefore desirable to minimise its use by keeping the beryllium content below this level.

Manganese (Mn) is an optional component of the alloys which may be included if there is a requirement for iron (Fe) removal. When Mn is a component it is preferably present in amounts less than 1%, more preferably less than 0.75%, more preferably 0.1-0.5%, more preferably 0.2-0.4% and most preferably about 0.3%. Other elements may also form optional components of the alloys provided that they do not adversely affect commercially significant properties of the alloys.

The presence of iron reduces corrosion resistance. Preferably, alloys of the present invention contain less than 100 ppm iron, more preferably less than 40 ppm iron, and most preferably substantially no iron.

The present inventor has ascertained that corrosion resistance decreases with decreasing aluminium content. All alloys of the present invention contain a minimum of 2.5% aluminium. Preferably, alloys of the present invention contain 2.5-5% aluminium, more preferably about 3-4.5% aluminium, and most preferably about 3.5-4% aluminium.

The present inventor has also ascertained that brittleness increases to the aluminium rich and zinc rich side of line CD.

The presence of nickel (Ni) reduces corrosion resistance. Preferably, alloys of the present invention contain less than 25 ppm nickel, more preferably less than 10 ppm nickel, and most preferably substantially no nickel.

The presence of silicon (Si) reduces corrosion resistance and mechanical properties. Preferably, alloys of the present invention contain less than 0.1% silicon, more preferably less than 0.08% silicon, and most preferably substantially no silicon.

In addition to resistance to ignition when molten, various preferred embodiments of the present invention exhibit one or more other commercially desirable properties such as recyclability, castability, resistance to hot cracking, corrosion resistance, creep resistance, low sound dampening coefficients and good surface finish.

A significant commercial impediment to the use of magnesium alloys is the waste which results from the difficulty of recycling so-called “returns” which include runners, biscuits etc from die casting. Typically, 30-70% of a diecasting consists of runners and biscuits that need to be recycled. Difficulties in the recycling of magnesium alloys are generally attributed to a significant amount of surface oxides which result in high melt losses in the form of dross and sludge. Generally, recycling is carried out in a separate operation in order to enable removal of oxides without entraining them in the melt and including them in subsequent diecastings. Surprisingly, the present inventor has ascertained that at least preferred embodiments of the alloys of the present invention have enhanced recyclability. Runners and other die casting scrap of alloys of the present invention have been successfully returned directly to melts without the need for any refining or purification. Without wishing to be bound by theory, the recyclability is believed to be closely related to the modification of oxidation behaviour which leads to suppression of ignition of molten alloys.

EXAMPLES Example 1

Magnesium alloys without beryllium additions and with various amounts of aluminium, zinc and calcium were melted at 700° C. under a sulphur hexafluoride (SF₆) containing protective atmosphere, then poured in air into a mould. The top surface of the resulting casting was left exposed to air. Four different types of behaviour were observed depending upon the composition.

Behaviour1—the surface of the casting initially turned black then ignited as illustrated in FIG. 3.

Behaviour2—the surface turned black but did not ignite as illustrated in FIG. 4.

Behaviour3—the surface was initially shiny then later ignited as illustrated in FIG. 5.

Behaviour4—the surface remained shiny with no ignition as illustrated in FIG. 6.

Table 1 lists the behaviour observed for a range of different alloys. The addition of more than 10% of zinc was sufficient to prevent burning and resulted in a blackened surface. Calcium additions without zinc produced a shiny surface, but 0.8% calcium was required to prevent ignition. The addition of calcium to alloys with sufficient zinc to prevent burning converted the surface to a shiny appearance with as little as 0.05% calcium producing a partially shiny surface. Increases in the calcium content lead to a progressive decrease in the amount of blackening. At 0.4% calcium no blackening was observed.

The alloys containing 10% zinc (see Table 1) turned black then ignited, while alloys with higher zinc contents did not ignite. The alloys were deliberately poured at high temperature (700° C.) to remove low temperature as a possible reason for absence of ignition. It is anticipated that commercial casting would occur at a temperature in the order of 30-40° C. lower with a consequent decrease in the propensity for ignition. TABLE 1 Behaviour of Molten Magnesium Alloys Exposed to Air Weight Weight Weight % Zn % Al % Ca Behaviour of exposed surface — — — Turned black then ignited 2.6 — — Turned black then ignited 5.2 — — Turned black then ignited 10.0 — — Turned black then ignited 18.8 — — Turned black, no ignition — 1.1 — Turned black then ignited — 2.2 — Turned black then ignited — 4.4 — Turned black then ignited — 8.8 — Turned black then ignited 2.6 1.1 — Turned black then ignited 5.2 2.2 — Turned black then ignited 10.0 4.4 — Turned black then ignited 14.4 6.0 — Turned black, no ignition 18.8 8.0 — Turned black, no ignition — — 0.05 Initially shiny then ignited — — 0.1 Initially shiny then ignited — — 0.2 Initially shiny then ignited — — 0.4 Initially shiny then ignited — — 0.8 Shiny, no ignition 18.8 8.0 0.05 Partially shiny, partially blackened, no ignition 18.8 8.0 0.1 Partially shiny, partially blackened, no ignition 18.8 8.0 0.2 Shiny with small area blackened, no ignition 18.8 8.0 0.4 Shiny, no ignition

Example 2

Additional melts were prepared and poured into a mould in the same manner as described above in Example 1. A metal scraper was then applied to the surface of the metal after pouring but while the metal was still molten.

FIG. 7 illustrates the behaviour of pure magnesium which oxidized so rapidly that it was not possible to expose shiny metal.

FIG. 8 illustrates the behaviour of a Mg-5% Zn alloy which also oxidized rapidly. Shiny metal could be exposed, but only for a small fraction of a second.

FIG. 9 illustrates the behaviour of a Mg-10% Zn alloy. The oxidation tendency was greatly reduced as indicated by the absence of “cauliflower-like” growths around the perimeter and the increase in shiny metal exposed.

FIGS. 10 and 11 illustrate the behaviour of Mg-15% Zn and Mg-20% Zn alloys respectively. In both cases it was relatively easy to expose shiny metal which took several seconds to re-oxidize. Neither formed “cauliflower-like” growths.

A further series of alloys was produced all containing 0.1% calcium and varying amounts of zinc. FIGS. 12, 13 and 14 show the appearance of the alloys immediately after pouring (FIGS. 12 a, 13 a and 14 a) then a short time (about 1 minute) later (FIGS. 12 b, 13 b and 14 b).

FIGS. 12 a and 12 b show the behaviour of a zinc free alloy. After initially appearing shiny this alloy developed “cauliflower-like” growths then later ignited.

FIGS. 13 a and 13 b show the behaviour of an alloy containing 5% zinc. This alloy also developed “cauliflower-like” growths and ignited, but at a slower rate than the zinc free alloy of FIG. 12.

FIGS. 14 a and 14 b show the behaviour of a 10% zinc alloy. In this alloy both the “cauliflower-like” growths and ignition were suppressed. The ultimate appearance after the sample was allowed to air cool to room temperature was unchanged from FIG. 14 b.

Example 3

Additional melts were prepared and poured into a mould in the same manner as described above in Example 1. The melts contained 13% zinc, 3.6% aluminium and varying amounts of beryllium and calcium. The calcium and beryllium contents of these alloys are given in Table 2. Alloys 1 and 6 were calcium-free and alloys 1-4 were beryllium-free. The final appearance of the castings is shown in FIG. 15. All of the alloys that contained some calcium or beryllium solidified with a shiny skin. Alloy 1 which was free of both calcium and beryllium solidified with a blackened skin. TABLE 2 Magnesium Alloy Compositions Alloy % Ca % Be 1 <0.01 <0.0001 2 0.04 <0.0001 3 0.10 <0.0001 4 0.19 <0.0001 5 0.19 0.0007 6 <0.01 0.0007 7 0.02 0.0008 8 0.05 0.0008 9 0.10 0.0010

It is to be clearly understood that although prior art publications are referred to herein, this reference does not constitute an admission that any of these documents forms part of the common general knowledge in the art in Australia or in any other country. 

1. An alloy consisting of zinc (Zn) and aluminum (Al) in amounts which fall within a quadrangle defined by lines AB, BC, CD, and DA wherein: A is 10% Zn-2.5% Al, B is 10% Zn-5% Al, C is 13% Zn-6.4% Al, and D is 19% Zn-2.5% Al; calcium (Ca) and/or beryllium (Be) in amounts which fall within a quadrangle defined by lines EF, FG, GH and HE wherein: E is 0.01% Ca-0% Be, F is 1% Ca-0% Be, G is 0% Ca-0.0025% Be, and H is 0% Ca-0.0001% Be optionally manganese; and the balance magnesium except for incidental impurities.
 2. An alloy as claimed in claim 1 containing greater than 11% zinc.
 3. An alloy as claimed in claim 2 containing 12-14% zinc.
 4. An alloy as claimed in claim 1 containing 2.5-5% aluminum.
 5. An alloy as claimed in claim 4 containing 3-4.5% aluminum.
 6. An alloy as claimed in claim 1 containing 0.01-0.5% calcium.
 7. An alloy as claimed in claim 6 containing 0.05-0.2% calcium.
 8. An alloy as claimed in claim 1 containing 0.0002-0.002% beryllium.
 9. An alloy as claimed in claim 8 containing 0.0005-0.001% beryllium.
 10. An alloy as claimed in claim 1 containing manganese in an amount less than 1%.
 11. An alloy as claimed in claim 10 containing 0.1-0.5% manganese.
 12. A magnesium based alloy consisting of: 11-13.5% zinc, 3-4.5% aluminum, 0.05-0.15% calcium, 0.0005-0.001% beryllium, optionally manganese in an amount less than 0.5%, and the balance being magnesium except for incidental impurities.
 13. A magnesium based alloy consisting of: 11.5-13.5% zinc, 3-4.5% aluminum, optionally manganese in an amount less than 0.5%, either 0.05-0.15% calcium, or 0.0005-0.001% beryllium, and the balance being magnesium except for the incidental impurities.
 14. A magnesium based alloy consisting of: 11.5-13.5% zinc, 3-4.5% aluminum, 0.2-0.4% manganese, 0.05-0.15% calcium, 0.0005-0.001% beryllium, and the balance being magnesium except for incidental impurities.
 15. A magnesium based alloy consisting of: 11.5-13.5% zinc, 3-4.5% aluminum, 0.2-0.4% manganese, either 0.05-0.15% calcium, or 0.0005-0.001% beryllium, and the balance being magnesium except for incidental impurities.
 16. An alloy as claimed in claim 12 containing 12-13% zinc, 3.5-4% aluminum, less than 0.08% silicon, less than 40 ppm iron, and less than 10 ppm nickel. 